Objective Lens?

Why are there an Objective Lens on some Scopes and for what reason?
I always set mine at 00, other times I set mine at 100 yards. Can this give me a relitive idea
in knowing the distance of my target?
I have no idea how to use this, and I am sure some others are as lost as I am.
Can this help me be a better shot by understanding how it works?
I have a few Scopes with this lens and I don't know why they are there.
Thanks Guys.

Don't rely on the numbers on the scope ring. If I set some of my scopes to the yardages I've ranged I still get paralax. I set my rifle up and slightly bob my head while I adjust the paralax out. You can adjust a side paralax turret without disturbing your rifle much. That's one reason I like the side adjustment instead of the adjustment on the front.

All traditional rifle scopes have an objective lens, it's the first lens that light passes through on its way to your eye. An adjustable objective (AO) simply moves that lens forward or backward to eliminate any parallax in the scope at a given distance. The numbers (in theory) should tell you where to set the ring to achieve parallax free shooting at those given distances but they are hardly ever correct.

LRSickle described how to accurately set your AO for a given distance by looking through your scope at the intended target, moving your head side to side or up and down, and then changing your AO until the crosshairs stay in the same place on the target regardless of where your eye is behind the scope.